In order to reduce CO2 as a countermeasure against global warming, the increase in the power generation amount and the yield of chemical industry products with respect to the usage amount of fossil fuels caused by the enhancement in the efficiency of a power generation boiler, a synthetic reactor in the chemical industry, and the like has become a problem to be solved urgently. To solve this problem, various kinds of products, which are heat and pressure-resistant parts, are required to have high-temperature heat resistance and corrosion resistance that are more excellent than before. As the material for the product used in such a harsh environment, a material of Ni-based alloy that is more excellent in high-temperature strength and high-temperature corrosion resistance must be used in place of the conventional steel materials.
Unfortunately, the conventional Ni-based alloy is remarkably inferior in workability and weldability at high temperatures, and exhibits a significant decrease in ductility during heating at high temperatures as compared with the existing steel materials. Therefore, for the above-described heat and pressure-resistant product, especially for a product having a large wall thickness and a large product size, if the conventional Ni-based alloy is used, the production and usage of product are restricted remarkably.
As a typical example of the large-sized heat and pressure-resistant product, there are cited a plate material having a thickness of 40 mm or larger and a tube and pipe having a large size. For example, the main steam pipe of a power generation boiler has a size of about 500 mm in outside diameter, 50 mm in wall thickness, and 6 m in length. When such a large-sized product is produced, problems described below arise because the product has a size larger than that of a small-sized product such as a heat exchanger tube and a heating furnace tube.
Since the size of the material before hot working is large, the heating time is long, and further in all processes of hot working, only a small degree of working in which the rolling reduction ratio is about 3 can be performed. Therefore, the crystal grains are coarsened to about 0 in austenite grain size number, so that the grain boundaries are liable to be affected by the segregation of P and S. Also, the cooling rate after hot working or welding decreases remarkably, and a brittle phase is liable to precipitate in the cooling process. Therefore, hot working cracks and flaws during the production and cracks caused by the restraint during welding occur easily. Also, faults such as cracks may be caused by the decrease in ductility during the long-term use of actual equipment or cracks during repair welding.
For example, the alloy 617 (Ni base-22Cr-9Mo-12Co-1Al—Ti—(Fe<1.5%)), which has conventionally been known widely as a Ni-based alloy, has been regarded as very likely as a material for a next-generation power generation boiler. Unfortunately, this alloy is expensive because of a large amount of Co contained therein. Also, this alloy cannot be put to practical use as a material for a large-sized product, and has merely been used practically as a material having a relatively small size. If such a large-sized product, for example, having a main steam pipe size is produced by using this alloy, significant cracks will occur during hot working, and cracks and breakage will be caused during bending and welding by hardening and a significant decrease in ductility due to the precipitation of γ′ phase. This is the reason why this alloy cannot be put to practical use as a material for a large-sized product.
Patent Document 1 discloses an austenitic stainless steel used at a steam temperature of 700° C. or higher and a producing method for the same. This steel is a material excellent in high-temperature strength and stability of microstructure. However, like the alloy 617, this steel has a fear that hot working cracks caused by low ductility during the production of a large-sized product or in the actual use of actual equipment may occur.
Patent Document 2 discloses a high-Cr austenitic heat-resistant alloy excellent in high-temperature strength and corrosion resistance. This alloy is a special material mainly aiming at precipitation strengthening caused by Cu-enriched phase and α-Cr phase by adding large amounts of Cu and Cr. As the product to which this alloy is applied, a heat exchanger tube and a heating furnace tube each having a relatively small size are assumed.
Patent Document 3 discloses a producing method for an austenitic heat-resistant steel tube excellent in high-temperature strength. As is apparent from the claims, this producing method is premised on cold rolling, so that this producing method is used for producing a small-sized steel tube. Cracks occurring during producing a large-sized steel tube and pipe, and cracks occurring during repair welding caused by the decrease in ductility when this steel tube and pipe is used for actual equipment are feared.
The invention disclosed in Patent Document 4 also relates to a small-sized superheater tube mainly developing corrosion resistance and strength at high temperatures, and therefore presents the same problems as described above. Further, Patent Document 5 and Patent Document 6 also disclose austenitic heat-resistant materials. These materials as well, like the above-described steel and the like, was developed mainly to provide high-temperature strength and high-temperature corrosion resistance, and was not developed by considering the improvement in workability and aging ductility of a large-sized product.